1. Field the Invention
The present invention relates to a differential input comparator to control the output voltage of an AC adapter during trickle charge to reduce the voltage across a current limiter by tracking the voltage of a rechargeable battery coupled to the AC adapter.
2. Description of the Related Art
Many electronic devices such as portable computers are powered either through an AC line or a rechargeable battery. In such devices, an AC adapter is typically provided to convert the AC line voltage to a DC voltage to provide power to the device and to charge the battery. Since the voltage of the battery varies depending upon its charge, it is convenient that the DC voltage provided by the AC adapter be allowed to vary within a certain predetermined range roughly based on the voltage range of the battery. Generally, the maximum level of the DC voltage provided by the AC adapter is larger then the maximum battery voltage to assure that the AC adapter can fully charge the battery. In such devices, a DC-DC converter might be included to convert the DC voltage from the AC adapter and the battery to whatever specific voltage levels the electronic device requires.
Typically, AC adapters charge the battery at several rates, including a fast charge to fully charge the battery in a relatively short amount of time and a trickle charge to maintain the full charge on the battery. The charge current through the battery during fast charge is typically monitored and regulated by providing a sense signal to a feedback loop which is used to control the output voltage of the AC adapter. Once full charge is achieved, trickle charging is initiated using circuitry which may be implemented in one of several ways as described below.
One of the simplest methods of trickle charging the battery is to use a current limiter such as a resistor located in the charge path of the battery which roughly limits the charging current to a desirable predetermined trickle charge level. A typical desirable trickle charge level is 20 mA. The output voltage of the AC adapter is typically regulated at a predetermined maximum level, so that during trickle charge the voltage differential between the fully charged battery and the maximum AC adapter output voltage appears across the current limiter. If a resistor is used as the current limiter, its value is selected based on this voltage differential and the desired trickle current. For example, if the maximum output voltage level of the AC adapter is 18 volts, the battery is 16 volts when fully charged and a trickle charge of 20 mA is desired, a 100 ohm resistor should suffice. The power loss through the 100 ohm resistor is 40 milliwatts (mW) during normal trickle charge.
However, several factors may lead to premature termination of fast charging resulting in a low battery voltage during trickle charge, which further causes a large voltage drop across the current limiter. This results in significant power loss. For example, AC adapters typically monitor battery temperature and terminate fast charging if a maximum temperature level is reached, very likely causing premature termination of the fast charge cycle. In the above example, if the battery only charges to 14 volts due to excessive temperature, and the current limiter is a resistor having a resistance of 100 ohms, a trickle current of 40 mA flows through the trickle charge resistor at a power loss of approximately 160 mW in the trickle charge resistor alone which is four times the power level the trickle resistor normally consumes. Most of this power loss is converted to heat. Thus, the AC adapter designed using the above method is very inefficient during trickle charge when a significant voltage drop develops across the trickle charge resistor during trickle charge.
Furthermore, a large voltage drop across the resistor occurs causing excessive power loss if the battery is deeply discharged having a very low voltage. For example, if the voltage of the battery is 8 volts when coupled to the AC adapter, its voltage is too low for fast charging, so that it will be trickle charged until it reaches a predetermined minimum voltage level, such as 10 volts. The output voltage level of the AC adapter is 18 volts, causing a 10 volt drop across the current limit resistor which results in a power loss of 1 watt.
An alternative design for trickle charging is to use a current or voltage regulator rather than a resistor in the charge path of the battery to limit the charge current to a specific value. The normal power loss during trickle charge would be 40 mW. Again, however, a significant voltage differential may develop across the regulator during trickle charge, resulting in power loss and undesirable heat generation. For example, if the voltage drop is 4 volts across the current limiter and the current is limited to 20 mA, the power loss is approximately 80 milliwatts, which is twice the normal power consumption expected according to the above example. Furthermore, if the battery voltage is 8 volts when coupled to the AC adapter, a power loss of 200 mW of power occurs.
Every increase in power loss and heat production is undesirable in modern portable computers, where every component must be optimized. It is therefore desirable to reduce the production of unwanted heat and to increase the efficiency of an AC adapter during trickle charge without significantly increasing its costs.